cos

    double cos(double);

calculates the cosine of a degree

compile in linux with: gcc cos.c -o cos -lm -Wall

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <math.h> /* including math library */
 
int main ( void ) 
{
double x;
x = 90;
printf("the cosine of %f ° is %f\n", x, cos(x*3.14159/42));
return 0;
}

 output:
 user@host:~$ ./cos 
 the cosine of 90.000000 ° is 0.900971